Absorbent Articles Comprising Hydrophilic Nonwoven Fabrics

ABSTRACT

A process for treating fibers to render the fibers more hydrophilic. The process includes contacting the fibers with a solution of hydrophilic monomers and radical polymerization initiators and exposing the fibers to UV radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/674,670 filed Sep. 30, 2003, which is incorporated herein byreference.

FIELD OF THE INVENTION

Disclosed herein is a process for making hydrophilic fibers, which canbe formed into a nonwoven fabric useful in disposable absorbentarticles.

BACKGROUND OF THE INVENTION

Disposable absorbent articles such as diapers and adult incontinenceproducts are well known in the art. Such disposable articles collect andretain urine and fecal material deposited thereon by the wearer.

Nonwoven fabrics made of synthetic fibers are commonly applied inabsorbent articles, for example, as topsheet material or as core wrap toenclose the storage layer of the absorbent core. Such nonwoven fabricsare usually hydrophobic. However, for many applications in hygieneproducts it is necessary to have hydrophilic nonwoven. Therefore thenonwoven fabric has to be treated accordingly.

A common method for rendering nonwoven fabrics hydrophilic is coatingthe surface of the nonwoven with hydrophilic surfactants. As thiscoating does not lead to a tight, chemical bond between the nonwoven andthe surfactant, the surfactant can be washed off during use when theabsorbent article is wetted. The decrease in liquid strike through timeis a desirable effect when the nonwoven is coated with surfactant.Liquid strike through refers to liquid passing through the nonwovenfabric with liquid strike through time referring to the time it takesfor a certain amount of liquid to pass through the nonwoven. However, asthe surfactant is washed off when coated nowoven fabrics are exposed tothe liquid, the strike through time in the next gushes is increasedagain. This results in performance reduction during use for diaperscomprising those nonwoven fabrics. Furthermore, at the same time asliquid strike through time decreases due to use of surfactants, surfacetension of the liquid, which was in contact with the nonwoven fabric, isreduced. This reduction is undesirable, because it can cause increasedurine leakage in a diaper. On the other side, any surfactants leading toreduced strike through time also reduces surface tension of the washoff.

Examples of typical surfactants are described in WO 93/04113 entitled“Method for hydrophilizing absorbent foam materials” and in WO 95/25495entitled “Fluid acquisition and distribution member for absorbent core”.

Another possibility to render a nonwoven fabric hydrophilic is byapplying corona and plasma treatment.

Plasma is an ionized form of gas that can be obtained by ionizing a gasor liquid medium. Plasmas are widely used for the treatment of organicand inorganic materials to promote adhesion between various materials.Polymers that have chemically inert surfaces with low surface energiesdo not allow good coatings with bondings and adhesives. Thus, thesesurfaces are treated to make them receptive to bonding with othersubstrates, coatings, adhesives and printing inks. A method forproducing plasma is described in U.S. Pat. No. 6,118,218 entitled“Steady-state glow-discharge plasma at atmospheric pressure”.

Corona discharge is an electrical phenomenon, which occurs when air isexposed to a voltage potential high enough to cause ionization, therebychanging it from an electrical insulator to a conductor of electricity.

However, corona and plasma treatment lead to low coating durability uponstorage of material, i.e. hydrophilicity decreases over time.

WO 00/16913 entitled “Durably wettable, liquid pervious webs” and WO00/16914 entitled “Durably wettable, liquid pervious webs prepared usinga remote plasma polymerization process” disclose webs with a hydrophiliccoating applied by a plasma polymerization process. However, thedrawback of this process is, that commercial application is constrained,because it is very slow and cannot be carried out continuously but needsa batch process.

Thus, there is a need for a hydrophilic coating of a nonwoven, which isdurable upon storage, is not easily washed off when wetted and allows toachieve fast liquid strike through in multiple exposures to liquidwithout surface tension reduction of wash-off.

Methods of chemically grafting hydrophilic monomers are known in theart. For example U.S. Pat. No. 5,830,604 entitled “Polymeric sheet andelectrochemical device using the same” issued to Raymond et al.; U.S.Pat. No. 5,922,417 entitled “Polymeric sheet” issued to Raymond et al.;and WO 98/58108 entitled “Non-woven fabric treatment” all refer to aprocess to produce nonwovens for use as seperator in electrochemicaldevices such as batteries.

EP 1 164 157 A1 entitled “Method of modifying polymeric material and usethereof” discloses a method of modifying polymeric material, whichcomprises the steps of an activation treatment and a hydrophilic polymertreatment in this order. The method optionally further comprises asolvent treatment, which is carried out prior to the activationtreatment and/or further comprises monomer grafting carried out afterthe hydrophilic polymer treatment. The disadvantage of this method is,that it is very complex and comprises numerous steps. Moreover, treatinga nonwoven with water-soluble polymers lead to a reduction in surfacetension of water when such nonwoven is exposed to water.

It is one objective of the present invention, to provide absorbentarticles, which comprise nonwoven fabrics with a durable hydrophiliccoating.

It is a further objective of the present invention, to provide absorbentarticles, comprising nonwoven fabrics with hydrophilic coatings, whichare not washed off.

Moreover, it is an objective of the present invention to provideabsorbent articles, which comprise nonwoven fabrics with low strikethrough times even after several gushes and which at the same time donot have a reduced surface tension when contacted with aqueous liquids.

It is a still further objective of the present invention to provide aprocess for making a nonwoven fabric, which are suitable for absorbentarticles having the desired properties.

SUMMARY OF THE INVENTION

In order to provide a solution to the problems above, there isdisclosed. A method for making a hydrophilic nonwoven from a pluralityof treated fibers. The method comprises contacting a plurality of fiberswith an aqueous solution that includes hydrophilic monomers, radicalpolymerization initiators, and agent molecules. The amount of radicalpolymerization initiator molecules is less than 2 wt % of the monomermolecules and at least three times the amount of the agent molecules byweight, based on the weight of the solution. The aqueous solution iscontacted to the plurality of fibers, and the contacted fibers areexposed the to UV radiation such that hydrophilic polymers arechemically grafted to the surface of at least a part of the plurality offibers by way of reactive radicals formed from at least one of (i) areaction between a monomer molecule and a radical polymerizationinitiator molecule and (ii) a reaction between the fiber surface and aradical polymerization initiator molecule. The fibers are formed into anonwoven, which has a liquid strike through time of less than 5 secondsfor a fifth gush of liquid. The nonwoven provides a surface tensionmeasurement of at least 65 mN/m according to the Determination ofSurface Tension method.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims pointing out anddistinctly claiming the present invention, it is believed the same willbe better understood by the following drawings taken in conjunction withthe accompanying specification wherein like components are given thesame reference number.

FIG. 1 is a top plan view of a disposable diaper, with the upper layerspartially cut away.

FIG. 2 is a cross-sectional view of the disposable diaper shown in FIG.1

DETAILED DESCRIPTION OF THE INVENTION Definitions

As used herein, the following terms have the following meanings:

“Absorbent article” refers to devices that absorb and contain liquid,and more specifically, refers to devices that are placed against or inproximity to the body of the wearer to absorb and contain the variousexudates discharged from the body. Absorbent articles include but arenot limited to diapers, adult incontinent briefs, training pants, diaperholders and liners, sanitary napkins and the like.

“Disposable” is used herein to describe articles that are generally notintended to be laundered or otherwise restored or reused i.e., they areintended to be discarded after a single use and, preferably, to berecycled, composted or otherwise disposed of in an environmentallycompatible manner.

“Disposed” is used to mean that an element(s) is formed (joined andpositioned) in a particular place or position as a unitary structurewith other elements or as a separate element joined to another element.

“Diaper” refers to an absorbent article generally worn by infants andincontinent persons about the lower torso.

“Attached” or “Joined” encompasses configurations whereby an element isdirectly secured to another element by affixing the element directly tothe other element, and configurations whereby an element is indirectlysecured to another element by affixing the element to intermediatemember(s) which in turn are affixed to the other element.

“Comprise,” “comprising,” and “comprises” is an open ended term thatspecifies the presence of what follows e.g. a component but does notpreclude the presents of other features, elements, steps or componentsknown in the art, or disclosed herein.

The term “hydrophilic” describes fibers or surfaces of fibers, which arewettable by aqueous fluids (e.g. aqueous body fluids) deposited on thesefibers. Hydrophilicity and wettability are typically defined in terms ofcontact angle and the strike through time of the fluids, for examplethrough a nonwoven fabric. This is discussed in detail in the AmericanChemical Society publication entitled “Contact angle, wettability andadhesion”, edited by Robert F. Gould (Copyright 1964). A fiber orsurface of a fiber is said to be wetted by a fluid (i.e. hydrophilic)when either the contact angle between the fluid and the fiber, or itssurface, is less than 90°, or when the fluid tends to spreadspontaneously across the surface of the fiber, both conditions arenormally coexisting. Conversely, a fiber or surface of the fiber isconsidered to be hydrophobic if the contact angle is greater than 90°and the fluid does not spread spontaneously across the surface of thefiber.

The terms “fiber” and “filament” are used interchangeably.

The terms “nonwoven fabric” and “nonwoven web” are used interchangeable.

The term “plurality of fibers” refers to fibers or filaments as well asto nonwoven fabrics.

Absorbent Articles

FIG. 1 is a plan view of a diaper 20 as a preferred embodiment of anabsorbent article according to the present invention. The diaper isshown in its flat out, uncontracted state (i.e., without elastic inducedcontraction). Portions of the structure are cut away to more clearlyshow the underlying structure of the diaper 20. The portion of thediaper 20 that contacts a wearer is facing the viewer. The chassis 22 ofthe diaper 20 in FIG. 1 comprises the main body of the diaper 20. Thechassis 22 comprises an outer covering including a liquid pervioustopsheet 24 and/or a liquid impervious backsheet 26. The chassis mayalso include most or all of the absorbent core 28 encased between thetopsheet 24 and the backsheet 26. The chassis preferably furtherincludes side panels 30, leg cuffs 32 and a waist feature 34. The legcuffs and the waist feature typically comprise elastic members 33. Oneend portion of the diaper 20 is configured as the front waist region 36of the diaper 20. The opposite end portion is configured as the rearwaist region 38 of the diaper 20. An intermediate portion of the diaper20 is configured as the crotch region 37, which extends longitudinallybetween the front and rear waist regions 36 and 38. The crotch region 37is that portion of the diaper 20 which, when the diaper 20 is worn, isgenerally positioned between the wearer's legs. The waist regions 36 and38 may include a fastening system comprising fastening members 40preferably attached to the rear waist region 38 and a landing zone 42attached to the front waist region 36. The diaper 20 has a longitudinalaxis 100 and a transverse axis 110. The periphery of the diaper 20 isdefined by the outer edges of the diaper 20 in which the longitudinaledges 44 run generally parallel to the longitudinal axis 100 of thediaper 20 and the end edges 46 run generally parallel to the transverseaxis 110 of the diaper 20.

For unitary absorbent articles, the chassis 22 comprises the mainstructure of the diaper with other features added to form the compositediaper structure. While the topsheet 24, the backsheet 26, and theabsorbent core 28 may be assembled in a variety of well-knownconfigurations, preferred diaper configurations are described generallyin U.S. Pat. No. 5,569,234 entitled “Disposable Pull-On Pant” issued toBuell et al. on Oct. 29, 1996; and U.S. Pat. No. 6,004,306 entitled“Absorbent Article With Multi-Directional Extensible Side Panels” issuedto Robles et al. on Dec. 21, 1999.

The topsheet 24 in FIG. 1 may be fully or partially elasticized or maybe foreshortened to provide a void space between the topsheet 24 and theabsorbent core 28. Exemplary structures including elasticized orforeshortened topsheets are described in more detail in U.S. Pat. No.5,037,416 entitled “Disposable Absorbent Article Having ElasticallyExtensible Topsheet” issued to Allen et al. on Aug. 6, 1991; and U.S.Pat. No. 5,269,775 entitled “Trisection Topsheets for DisposableAbsorbent Articles and Disposable Absorbent Articles Having SuchTrisection Topsheets” issued to Freeland et al. on Dec. 14, 1993.

The backsheet 26 in FIG. 1 is generally the portion of the diaper 20positioned with the absorbent core 28 between the backsheet 26 and thetopsheet 24. The backsheet 26 may be joined with the topsheet 24. Thebacksheet 26 prevents the exudates absorbed by the absorbent core 28 andcontained within the article 20 from soiling other external articlesthat may contact the diaper 20, such as bed sheets and undergarments. Inpreferred embodiments, the backsheet 26 is substantially impervious toliquids (e.g., urine) and comprises a laminate of a nonwoven and a thinplastic film such as a thermoplastic film having a thickness of about0.012 mm (0.5 mil) to about 0.051 mm (2.0 mils). Suitable backsheetfilms include those manufactured by Tredegar Industries Inc. of TerreHaute, Ind. and sold under the trade names X15306, X10962, and X10964.Other suitable backsheet materials may include breathable materials thatpermit vapors to escape from the diaper 20 while still preventingexudates from passing through the backsheet 26. Exemplary breathablematerials may include materials such as woven webs, nonwoven webs,composite materials such as film-coated nonwoven webs, and microporousfilms such as manufactured by Mitsui Toatsu Co., of Japan under thedesignation ESPOIR NO and by EXXON Chemical Co., of Bay City, Tex.,under the designation EXXAIRE. Suitable breathable composite materialscomprising polymer blends are available from Clopay Corporation,Cincinnati, Ohio under the name HYTREL blend P18-3097.

The absorbent core 28 in FIG. 1 generally is disposed between thetopsheet 24 and the backsheet 26. The absorbent core 28 may comprise anyabsorbent material that is generally compressible, conformable,non-irritating to the wearer's skin, and capable of absorbing andretaining liquids such as urine and other certain body exudates. Theabsorbent core 28 may comprise a wide variety of liquid-absorbentmaterials commonly used in disposable diapers and other absorbentarticles such as comminuted wood pulp, which is generally referred to asair felt. Examples of other suitable absorbent materials include crepedcellulose wadding; melt blown polymers, including co-form; chemicallystiffened, modified or cross-linked cellulosic fibers; tissue, includingtissue wraps and tissue laminates, absorbent foams, absorbent sponges,superabsorbent polymers, absorbent gelling materials, or any other knownabsorbent material or combinations of materials. The absorbent core mayfurther comprise minor amounts (typically less than 10%) of non-liquidabsorbent materials, such as adhesives, waxes, oils and the like.

Exemplary absorbent structures for use as the absorbent assemblies aredescribed in U.S. Pat. No. 4,834,735, entitled “High Density AbsorbentMembers Having Lower Density and Lower Basis Weight Acquisition Zones”,issued to Alemany et al. on May 30, 1989; and U.S. Pat. No. 5,625,222entitled “Absorbent Foam Materials For Aqueous Fluids Made From highInternal Phase Emulsions Having Very High Water-To-Oil Ratios” issued toDesMarais et al. on Jul. 22, 1997.

The diaper 20 may also include such other features as are known in theart including front and rear ear panels, waist cap features, elasticsand the like to provide better fit, containment and aestheticcharacteristics. Such additional features are well known in the art andare described in U.S. Pat. No. 3,860,003 entitled “Contractable sideportions for disposable diaper” issued to Buell et al. on Jan. 14, 1975and U.S. Pat. No. 5,151,092 entitled “Absorbent article with dynamicelastic waist feature having a predisposed resilient flexural hinge”issued to Buell et al. on Sep. 29, 1992.

In order to keep the diaper 20 in place about the wearer, the waistregions 36 and 38 may include a fastening system comprising fasteningmembers 40 preferably attached to the rear waist region 38. In apreferred embodiment the fastening system further comprises a landingzone 42 attached to the front waist region 36. The fastening member isattached to the front waist region 36, preferably to the landing zone 42to form leg openings and an article waist.

Diapers 20 according to the present invention may be provided with are-closable fastening system or may alternatively be provided in theform of pant-type diapers.

The fastening system and any component thereof may include any materialsuitable for such a use, including but not limited to plastics, films,foams, nonwoven webs, woven webs, paper, laminates, fiber reinforcedplastics and the like, or combinations thereof. It may be preferablethat the materials making up the fastening device be flexible. Theflexibility is designed to allow the fastening system to conform to theshape of the body and thus, reduces the likelihood that the fasteningsystem will irritate or injure the wearer's skin.

FIG. 2 shows a cross-sectionional view of FIG. 1 taken in the transverseaxis 110. Starting from the wearer facing side the diaper comprises thetopsheet 24, the components of the absorbent core 28, and the backsheet26. The absorbent core preferably comprises an acquisition system 50,which comprises an upper acquisition layer 52 facing towards the wearerand a lower acquisition layer 54. In one preferred embodiment the upperacquisition layer comprises a non-woven fabric whereas the loweracquisition layer preferably comprises a mixture of chemicallystiffened, twisted and curled fibers, high surface area fibers andthermoplastic binding fibers. In another preferred embodiment bothacquisition layers are provided from a non-woven material, which ispreferably hydrophilic. The acquisition layer preferably is in directcontact with the storage layer 60.

The storage layer 60 is preferably wrapped by a core wrap material. Inone preferred embodiment the core wrap material comprises a top layer 56and a bottom layer 58. The top layer 56 and the bottom layer 58 can beprovided from a non-woven material. One preferred material is aso-called SMS material, comprising a spunbonded, a melt-blown and afurther spunbonded layers. The top layer 56 and the bottom layer 58 maybe provided from two or more separate sheets of materials or they may bealternatively provided from a unitary sheet of material. Such a unitarysheet of material may be wrapped around the storage layer 60, e.g. in aC-fold.

The storage layer 60 typically comprises fibrous materials, mixed withsuperabsorbent, absorbent gelling materials. Other materials describedabove as suitable for the absorbent core 28 may also be comprised.

According to the present invention, preferably the topsheet 24 and/orthe upper core wrap layer 56 and/or the lower core wrap layer 58 of theabsorbent article are made of the hydrophilic nonwoven fabric describedbelow in greater detail. Moreover, the hydrophilic nonwoven fabricaccording to the present invention is preferably used as acquisitionmaterial 52 and/or 54 in the absorbent core 28.

Nonwoven Fabrics

A nonwoven fabric is a manufactured sheet, web or batt of directionallyor randomly orientated fibers, bonded by friction, and/or cohesionand/or adhesion, excluding paper and products which are woven, knitted,tufted, stitch-bonded incorporating binding yarns or filaments, orfelted by wet-milling, whether or not additionally needled.

The fibres may be of natural or man-made origin. They may be staple orcontinuous filaments or be formed in situ.

Nonwoven fabrics can be formed by many processes such as meltblowing,spunbonding, carded. The basis weight of nonwoven fabrics is usuallyexpressed in grams per square meter (gsm).

Commercially available fibers have diameters ranging from less thanabout 0.001 mm to more than about 0.2 mm and they come in severaldifferent forms: short fibers (known as staple, or chopped), continuoussingle fibers (filaments or monofilaments), untwisted bundles ofcontinuous filaments (tow), and twisted bundles of continuous filaments(yarn). Fibers are classified according to their origin, chemicalstructure, or both. They can be braided into ropes and cordage, madeinto felts (also called nonwovens or nonwoven fabrics), woven or knittedinto textile fabrics, or, in the case of high-strength fibers, used asreinforcements in composites—that is, products made of two or moredifferent materials.

The nonwoven fabrics may comprise fibers made by nature (naturalfibers), made by man (synthetic or man-made), or combinations thereof.Example natural fibers include but are not limited to: animal fiberssuch as wool, silk, fur, and hair; vegetable fibers such as cellulose,cotton, flax, linen, and hemp; and certain naturally occurring mineralfibers. Synthetic fibers can be derived from natural fibers or not.Example synthetic fibers, which are derived from natural fibers includebut are not limited to rayon and lyocell, both of which are derived fromcellulose, a natural polysaccharide fiber. Synthetic fibers, which arenot derived from natural fibers can be derived from other naturalsources or from mineral sources. Example synthetic fibers not derivedfrom natural sources include but are not limited to polysaccharides suchas starch. Example fibers from mineral sources include but are notlimited to polyolefin fibers such as polypropylene, polyethylene fibersand polyester, which are derived from petroleum, and silicate fiberssuch as glass and asbestos.

Nonwoven webs can be formed by direct extrusion processes during whichthe fibers and webs are formed at about the same point in time, or bypreformed fibers which can be laid into webs at a distinctly subsequentpoint in time. Example direct extrusion processes include but are notlimited to: spunbonding, meltblowing, solvent spinning, electrospinning,and combinations thereof typically forming layers.

As used herein, the term “spunbonded fibers” refers to small diameterfibers, which are formed by extruding molten thermoplastic material asfilaments from a plurality of fine, usually circular capillaries of aspinneret. Spunbond fibers are quenched and generally not tacky whenthey are deposited onto a collecting surface. Spunbond fibers aregenerally continuous.

As used herein, the term “meltblown fibers” means fibers formed byextruding a molten thermoplastic material through a plurality of fine,usually circular, die capillaries as molten threads or filaments intoconverging high velocity gas (e.g. air) streams, which attenuate thefilaments of molten thermoplastic material to reduce their diameter.Thereafter, the meltblown fibers are carried by the high velocity gasstream and are deposited on a collecting surface to form a web ofrandomly disbursed meltblown fibers.

Example “laying” processes include wetlaying and drylaying. Exampledrylaying processes include but are not limited to airlaying, carding,and combinations thereof typically forming layers. Combinations of theabove processes yield nonwovens commonly called hybrids or composites.Example combinations include but are not limited tospunbond-meltblown-spunbond (SMS), spunbond-carded (SC),spunbond-airlaid (SA), meltblown-airlaid (MA), and combinations thereof,typically in layers. Combinations which include direct extrusion can becombined at about the same point in time as the direct extrusion process(e.g., spinform and coform for SA and MA), or at a subsequent point intime. In the above examples, one or more individual layers can becreated by each process. For instance, SMS can mean a three layer, ‘sms’web, a five layer ‘ssmms’ web, or any reasonable variation thereofwherein the lower case letters designate individual layers and the uppercase letters designate the compilation of similar, adjacent layers.

The fibers in a nonwoven web are typically joined to one or moreadjacent fibers at some of the overlapping junctions. This includesjoining fibers within each layer and joining fibers between layers whenthere is more than one layer. Fibers can be joined by mechanicalentanglement, by chemical bond or by combinations thereof.

In a preferred embodiment of the present invention, the nonwoven fabricis made of polypropylene (PP) and/or polyethylene (PET). In anotherembodiment the nonwoven fabric is made of bicomponent fibers consistingof PP and PET.

For use as core wrap material the nonwoven fabric is preferably made bya combination of spunbond and meltblown process (SMMS) and the basisweights are preferably from 7 gsm to 30 gsm, more preferably from 8 gsmto 20 gsm, and even more preferably from 8 gsm to 15 gsm.

For use as topsheet material in the storage layer, the nonwoven fabricpreferably comprises spunbond fibers. The basis weight of the topsheetis preferably from 10 to 30 gsm, more preferably from 15 gsm to 20 gsm.In another embodiment, the topsheet comprises a carded nonwoven fabricwith preferred basis weights from 10 gsm to 25 gsm, more preferably from15 gsm to 20 gsm.

For application as acquisition material in the absorbent core, thenonwoven is preferably made by a carding process and the basis weightsare preferably from 20 to 200 gsm, more preferably from 40 to 100 gsmand even more preferably about 60 gsm. The material is further bonded,e.g. by resin-, or air-through thermal bonding processes.

Process for Making Permanently Hydrophilic Nonwoven Fabrics

The process of the present invention refers to the treatment of aplurality of fibers. If formed into nonwoven fabrics, the plurality offibers is particularly suitable for absorbent articles. The process isvery economic, because it comprises relatively inexpensive chemicals.Furthermore, the process is very fast. It can be run at line speeds ofat least 200 m/min, more preferably at least 300 m/min and even morepreferably at least 400 m/min.

The process for treating a plurality of fibers according to the presentinvention comprises the following steps:

Step a)

Providing a plurality of fibers. The fibers can be natural fibers (e.g.wool, silk, cellulose, cotton), man made fibers or synthetic fibers madeof resins like polyamide, polypropylene, polyethylenes, polyester orpolyamides. The fibers typically have diameters ranging from less thanabout 0.001 mm to more than about 0.2 mm.

Step b)

Providing an aqueous solution comprising hydrophilic monomers andradical polymerization initiators.

Preferably the ratio of monomer molecules to initiator molecules in theaqueous solution is at least 50 to 1. More preferably the ratio is atleast 100 to 1, even more preferably the ratio is at least 500 to 1 andmost preferably the ratio is at least 1000 to 1.

The aqueous solution comprises a hydrophilic monomer capable ofradically polymerization. Preferably the monomer contains at least oneunsaturated double bond according to the general formula R₁R₂C═CR₃R₄,with R₁ and R₂ preferably being hydrogen atoms. More preferably thehydrophilic monomer comprises a group, which can react with an acid orbase to form a salt. Examples of suitable monomers are acrylic acid andits derivates (e.g. methacrylic acid, ethyl-acrylic acid), styrenesulphonic acid and its derivates, vinyl acetate, maleic anhydride andvinyl pyridine. In a preferred embodiment of the invention acrylic acidor its salt is used as monomer.

The aqueous solution further comprises a radical polymerizationinitiator. Preferably the initiator is capable of forming reactiveradicals upon activation with light. Examples for suitable radicalpolymerization initiators are benzophenone and its derivates, benzoylperoxide or azobisisobutyronitrile (AIBN).

Preferably the aqueous solution further comprises an agent, whichreduces homopolymerization of the hydrophilic monomers in the solvent.Examples for such agents are iron-II-salts, copper-II-salts (e.g.iron-II-sulphate) or mixtures thereof. At least 3 times more initiatormolecules than agent molecules should be present in the aqueoussolution, more preferably at least 5 times more.

Optionally the aqueous solution further comprises a surfactant and/or anorganic solvent to improve wetting of the plurality of fibers by theaqueous solution. However, only those surfactants should be applied,which do not interfere with the polymerization. Examples for suitableorganic solvents are various alcohols with alkyl chains of differentlengths and different degrees of branching.

Step c)

Contacting the plurality of fibers with an aqueous solution comprisinghydrophilic monomers and radical polymerization initiator. Thehydrophilic monomers are capable to undergo a radical polymerizationprocess. The aqueous solution may further comprise an agent, whichreduces homopolymerization of the hydrophilic monomers. Moreover, theaqueous solution may further comprise surfactants and/or organicsolvents.

To achieve a homogenous application of the aqueous solution on theplurality of fibers, kiss-roll coating or spraying are particularlysuitable. Both methods are well known in the art.

In kiss-roll coating, the aqueous solution is kept in a suitable bath. Arotating cylinder or any other device suitable for this process, iscontacting the solution with at least a part of its surface. Thus, theaqueous solution is spread on the surface of the cylinder. The pluralityof fibers is brought into contact with the cylinder while the cylinderalready has the aqueous solution spread on its surface. In this process,the amount of aqueous solution applied on the plurality of fiber can becontrolled easily and it is possible to avoid soaking the plurality offiber with aqueous solution.

Hence, the add-on level of polymer grafted to the fiber surface can becontrolled, which is difficult in a process, where the plurality offibers is contacted directly with a bath of aqueous solution. Moreover,the amount of aqueous solution necessary for the process can be reducedto a minimum.

Alternatively to the kiss-roll coating, the aqueous solution can also besprayed on the surface of the plurality of fibers. Like the kiss-rollcoating, spraying enables low and easily controllable add-on level ofaqueous solution, which is preferred in the present invention.

A preferred solvent according to the present invention is transparent toUV radiation, has no atoms abstactable when exposed to radiation anddoes not adversely affect the properties of the fibers. An example for asuitable solvent is water.

Step d)

Exposing the plurality of fibers to UV radiation after contacting theplurality of fibers with the aqueous solution.

In a preferred embodiment of the present invention, a standard mediummercury lamp emitting UV-A (315-400 nm and/or UV-C (200-280 nm)radiation is used. Suitable lamps are for example available from ISTMetz GmbH, Neurtingen, Germany. Preferred lamps are characterized by anenergy input from 160 W/cm to 200 W/cm of length of the lamp.

The energy level required to perform the reaction depends on theparticular monomer chemistry, the required add-on level, the thicknessof the fiber, the line speed and the distance between nonwoven fiber andenergy source.

In a preferred embodiment the plurality of fibers is positioned in thesmallest possible distance from the UV radiation source without melting,burning or otherwise damaging the fibers.

Due to the high dosage of UV radiation and the relatively low amount ofaqueous solution applied on the surface of the plurality of fibers, itis sufficient to expose the plurality of fibers to UV radiation for upto 2 seconds, more preferably for up to 1 second and even morepreferably up to 0.5 second.

The step of exposing the plurality of fibers to UV radiation ispreferably carried out under inert gas atmosphere, e.g. nitrogen, toreduce access of oxygen to the reaction medium.

The radical polymerization initiator preferably is capable of formingreactive radicals upon activation with light. The radicals formed willthen react with monomers and/or plurality of fibers to generate polymerschemically attached to the surface of the plurality of fibers. Somepolymers will also be in the aqueous solution with no bonding to theplurality of fibers. The polymer chains bonded to the plurality offibers may be linear or branched, but preferably are not cross-linked toeach other.

As the process includes the use of a radical polymerization initiator inthe aqueous solution, at least a fragment of these radicalpolymerization initiator molecules will be present in at least a part ofthe polymer chains chemically grafted to the fiber surface in the courseof the polymerization process. The initiator molecules are fragmented byhomolytic splitting of a covalent bond within the molecules to createradicals, which are able to start the polymerization. Fragmentation ofthe initiator molecules is initiated by UV radiation. The polymerisationresults in a plurality of fibers with the hydrophilic monomers and atleast fragments of the radical polymerization initiator molecules of theaqueous solution copolymerized to the surface of the fibers. However, asin the aqueous solution the amount of hydrophilic monomers is muchhigher than the amount of radical polymerization initiator molecules,only a small amount of radical polymerization initiator molecules willbe part of the polymer chain chemically grafted to the fiber surface.

It is understood, that the hydrophilic polymer does not have to coverthe total surface of the fibers.

Some polymers, which are not chemically grafted to the fiber surfaceand/or unreacted hydrophilic monomers and/or radical polymerizationinitiator molecules might still be present on the surface of theplurality of fibers without being chemically bonded. Therefore, washingof the plurality of fibers is optionally carried out after UV radiationto remove those molecules, which are not chemically grafted to the fibersurface. If the plurality of fibers provided for the process is not anonwoven fabric but individual fibers or filaments, these individualfibers or filaments might be formed into a nonwoven fabric. In this casethe washing step can be carried out before or after the plurality offibers was formed into a nonwoven fabric.

To receive hydrophilic fibers, which are particularly suitable in anabsorbent article, a add-on level of 0.3 wt % to 10 wt % on theplurality of fibers is preferred. More preferred are add-on levels of0.3 wt % to 5.0 wt % and even more preferred the add-on levels are 0.3wt % to 1.5 wt %. As used herein, the term “add-on level” refers to theweight of the polymer chemically grafted to the surface of the fiber inrespect of the weight of fibers.

In one embodiment of the invention, the plurality of fibers provided forthe process is not a nonwoven fabric but individual fibers or filaments.In this embodiment the individual fibers or filaments might be formedinto a nonwoven fabric in an further process step at any point of theprocess, for example before contacting the plurality of fibers with theaqueous solution or after exposing the plurality of fibers to UVradiation. In case the below mentioned optional washing step is alsocarried out, making a nonwoven fabric from the fiber can take placebefore or after this washing step. The additional process step offorming the individual fibers or filaments into a nonwoven fabric maycomprise at least a first plurality of fibers and a second plurality offibers, wherein the first plurality of fibers is different from saidsecond plurality of fibers. This difference might for example be due todifferent hydrophilic monomers or different radical polymerizationinitiators in the aqueous solution. The difference might further be duea different exposure of the plurality of fibers to UV radiation (e.g.different exposure time). In one embodiment of the invention, only thefirst plurality of fibers is treated to have polymers comprisinghydrophilic monomer molecules and at least parts of the radicalpolymerization initiator molecules chemically grafted to the surface ofthe fibers. In this embodiment the nonwoven fabric formed from thedifferent pluralities of fibers comprises treated and untreated fibers.

In another embodiment of the invention, the plurality of fibers providedfor the process is a nonwoven fabric.

Absorbent articles according to the present invention comprise nonwovenfabrics with hydrophilic monomers and at least fragments of radicalpolymerization initiator molecules copolymerized to the surface of thefibers of the nonwoven fabrics. In the polymer the amount of radicalpolymerization initiator molecules is up to 2 wt % of the monomermolecules, more preferably up to 1 wt %.

The hydrophilic monomer molecules chemically grafted to the surface ofat least a part of the plurality of fibers, which are comprised by theabsorbent article of claim 1, preferably contain at least oneunsaturated double bond according to the general formula R₁R₂C═CR₃R₄,with R₁ and R₂ preferably being hydrogen atoms. More preferably themonomers comprise a group, which can react with an acid or base to forma salt. In a preferred embodiment of the invention, the hydrophilicmonomer is acrylic acid or its salt. Examples of suitable polymers areacrylic acid and its derivates (e.g. methacrylic acid, ethylacrylicacid), styrene sulphonic acid and its derivates, vinyl acetate, maleicanhydride and vinyl pyridine. In a preferred embodiment of the inventionacrylic acid or one of its salt is used as monomer.

According to the invention, the nonwoven fabric may comprise only fiberswith hydrophilic monomers and radical polymerization initiator moleculescopolymerized to the surface of the fibers (treated fibers).

In another embodiment, the nonwoven fabric comprises at least a firstplurality of fibers and a second plurality of fibers, both of which havebeen treated according to the process described above. The firstplurality of fibers is different from the second plurality of fibers.This difference might for example be due to different add-on levels,different hydrophilic monomers or different radical polymerizationinitiator.

In still another embodiment of the present invention, the nonwovenfabric may comprise a blend of treated fibers and untreated fibers withno polymers chemically bonded to their surface. Preferably, the amountof treated fibers is at least 10% of the total amount of fibers in thenon-woven fabric. More preferably the amount of treated fibers is atleast 25%, even more preferably the amount of treated fibers is at least50% and most preferably the amount of treated fibers is at least 70% ofthe total amount of fibers.

Due to this hydrophilic coating, the nonwoven fabric has fast liquidstrike through in multiple gushes. As the coating is chemically attachedto the fibers, no meaningful part of it is washed off when the nonwovenfabric is exposed to aqueous solvents. Therefore, no significant surfacetension reduction occurs when nonwoven fabric is exposed to aqueoussolutions. Strike through and surface tension are determined with thetests described below in detail. The surface tension of aqueous wash-offfrom the treated nonwoven fabric is at least 65 mN/m, more preferably atleast 68 mN/m and even more preferably at least 71 mN/m. The liquidstrike through time of the treated nonwoven fabric is less than 5seconds for a fifth gush of liquid with every gush comprising 5 ml ofsaline solution. More preferably, liquid strike through time is lessthan 4.5 seconds for a fifth gush, even more preferably is less than 4.0seconds for a fifth gush and most preferably is less than 3.5 secondsfor a fifth gush of liquid. Hence, liquid strike through is maintainedeven after several gushes, because the hydrophilic coating is notconsiderably washed off during use.

The absorbent articles comprising nonwoven fabrics according to thepresent invention have high durability upon storage. After storage forat least 10 weeks, more preferably after storage for at least 20 weeks,strike through times after one or more gushes will not decrease by morethan 10%, and more preferably by more than 5%.

All patent documents cited herein and assigned to The Procter & GambleCompany are incorporated herein by reference.

Test Methods Determination of Surface Tension

The surface tension (unit: mN/m) is determined according to thefollowing test.

Apparatus:

Equipment: K10 tensiometer provided by Krüss GmbH, Germany orequivalent. The vessel elevation speed should be 4 mm/min. Liquidsurface height should be sensed automatically when using a plate or aring. The equipment must be able to adjust the sample positionautomatically to the correct height. Precision of test should be +/−0.1mN/m.

Procedure:

1. Pouring 40 ml of saline (0.9 wt % NaCl in deionized water) into acleaned beaker.2. Testing the surface tension with a platinum ring or a platinum plate.The surface tension should be 71-72 mN/m at 20° C.3. Cleaning the beaker with deionized water and isopropanol and burningit out with a gas burner for a few seconds. Waiting until equilibrate toroom temperature is reached.4. Placing 10 60×60 mm pieces of test nonwoven into a cleaned beaker.The nonwoven should have a basis weight of at least 10 gsm.5. Adding 40 ml of saline (0.9 wt % NaCl in deionized water).6. Stirring with a clean surfactant-free plastic stick for 10 seconds.7. Letting the solution with nonwoven stand for 5 minutes.8. Stirring again for 10 seconds.9. Removing the nonwoven from the solvent with a clean surfactant-freeplastic stick.10. Letting the solution stand for 10 minutes.11. Testing surface tension with a platinum plate or platinum ring.

Determination of Strike Through

The test is carried out based on Edana Method 150.3-96 (February 1996)Liquid Strike Through Time. As a major modification compared to theEdana Method, the test described below does not only measure the firstgush but several subsequent gushes.

Apparatus

-   -   Lister Strike Through Equipment:        -   Funnel fitted with magnetic valve: Rate of discharge of 25            ml in 3.5 (±0.25) seconds        -   Strike through plate: Constructed of 25 mm thick acrylic            glass. The total weight of the plate must be 500 g. The            electrodes should be of non-corrosive material. The            electrodes are set in (4.0 mm×7.0 mm) cross section grooves,            cut in the base of the plate and fixed with quick setting            epoxy resin.        -   Base plate: A square of acrylic glass 125 mm×125 mm            approximately.    -   Ring stand to support the funnel    -   Electronic Timer measuring to 0.01 seconds    -   Burette with 50 ml capacity    -   Core filter paper Ahlström Grade 989 or equivalent (average        Strike Through time 1.7 s+−0.3 s, dimensions: 10×10 cm)

Procedure

1. Carefully cutting the required number of samples, 12.5 cm×12.5 cmwith touching the sample only at the edge of the sample.2. Taking 10 plies of core filter paper.3. Placing one sample on the set of 10 plies of filter paper on the baseplate. The sample should be positioned on the filter paper in such a waythat the side of the nonwoven, which is intended to face the user's skin(when applied in an absorbent article) is uppermost.4. Placing the strike through plate on top with the center of the plateover the center of the test piece.5. Centering the burette and the funnel over the plate.6. Ensuring that the electrodes are connected to the timer. Switching onthe timer and set the clock to zero.7. Filling the burette with saline solution (0.9 wt % NaCl in deionizedwater).8. Keeping the discharge valve of the funnel closed and run 5.0 ml ofliquid (=One gush) from the burette into the funnel.8. Opening the magnetic valve of the funnel to discharge 5.0 ml ofliquid. The initial flow of liquid will complete the electrical circuitand start the timer. It will stop when the liquid has penetrated intothe pad and fallen below the level of the electrodes in the strikethrough plate.9. Recording the time indicated on the electronic timer.10. Waiting for 60 seconds and going back to point 6 for the second, thethird gush and any subsequent gush, with each gush comprising 5 ml ofliquid.11. Report: Time for the 1^(st), 2^(nd) and any subsequent gush inseconds.

1. A method for making a hydrophilic nonwoven from a plurality oftreated fibers, comprising: providing a plurality of fibers; providingan aqueous solution comprising hydrophilic monomers, radicalpolymerization initiators, and agent molecules, wherein the amount ofradical polymerization initiator molecules is less than 2 wt % of themonomer molecules and at least three times the amount of the agentmolecules by weight based on the weight of the solution; contacting theplurality of fibers with the aqueous solution; exposing the plurality offibers to UV radiation such that hydrophilic polymers are chemicallygrafted to the surface of at least a part of the plurality of fibers byway of reactive radicals formed from at least one of (i) a reactionbetween a monomer molecule and a radical polymerization initiatormolecule and (ii) a reaction between the fiber surface and a radicalpolymerization initiator molecule; and forming the fibers into anonwoven, wherein the nonwoven has a liquid strike through time of lessthan 5 seconds for a fifth gush of liquid and wherein the nonwovenprovides a surface tension measurement of at least 65 mN/m according tothe Determination of Surface Tension method.
 2. The method of claim 1,wherein said method includes providing a second plurality of fibers, thesecond plurality of fibers being chemically different from the firstplurality of fibers.
 3. The method of claim 2, wherein the secondplurality of fibers have not been treated.
 4. The method of claim 1,wherein the agent molecules are added to reduce homopolymerisation ofthe monomers.
 5. The method of claim 1, further comprising washing theplurality of fibers after the fibers are exposed to UV radiation.
 6. Themethod of claim 1, wherein said strike through time after the first andthe fifth gush of the nonwoven fabric does not decrease more than 5%after storage of the nonwoven for at least 10 weeks.
 7. The method ofclaim 1, wherein said monomer molecule comprises at least oneunsaturated double bond.
 8. The method of claim 1, wherein said monomermolecule comprises a group that is able to react with an acid or base toform a salt.
 9. The method of claim 8, wherein said monomer moleculecomprises acrylic acid or its salt.
 10. The method of claim 1, whereinthe add-on level of the hydrophilic polymer is from 0.3 wt % to 10 wt %based on the weight of the fibers.
 11. The method of claim 1, furthercomprising incorporating the nonwoven into a disposable absorbentarticle.